Positively regulates a late step in synaptic vesicle exocytosis. Organizes the SNAREs into a cross-linked zigzag topology that, when interposed between the vesicle and plasma membranes, is incompatible with fusion, thereby preventing SNAREs from releasing neurotransmitters until an action potential arrives at the synapse. Also involved in glucose-induced secretion of insulin by pancreatic beta-cells. Belongs to the complexin/synaphin family. Note: This description may include information from UniProtKB.